Dorel Banabic

Non-quadratic yield criterion for orthotropic sheet metals under plane-stress conditions

The paper presents a new yield criterion for orthotropic sheet metals under plane-stress conditions. The criterion is derived from the one proposed by Barlat and Lian (Int. J. Plasticity 5 (1989) 51). Three additional coefficients have been introduced in order to allow a better representation of the plastic behaviour of the sheet metals. The predictions of the new yield criterion are compared with the experimental data for an aluminium alloy sheet and a steel sheet.

Prediction of the influence of yield locus on the limit strains in sheet metals

The paper presents a new approach to the Forming Limit Diagram (FLD) modeling. The yield criterion (1993), recently proposed by Hill, is used for the calculation of the limit strains in connection with the Swift’s instability condition for diffuse necking and by using the Marciniak–Kuczynski analysis. The influence of the ratio between uniaxial and biaxial yield stress (σu/σb), a, upon the FLD’s is presented.

Bulge testing under constant and variable strain rates of superplastic aluminium alloys

The present paper deals with superplastic forming of aluminium alloy AA5083 sheet metals tested at specific strain rates, temperatures and counter pressures by means of bulge testing using circular and elliptical dies and by the cone-cup testing method. Further, differences from batch to batch can lead to a different strain rates at the maximum m value. It is shown by experimental investigations that pulsating strain rates can lead to higher m values and to increased thickness strains.

The performance of Yld96 and BBC2000 yield functions in forming limit prediction

Abstract The objective of the work presented here is to assess the performance of two non-quadratic yield functions for orthotropic sheet metals under plane stress conditions, namely Yld96 and BBC2000, on forming limit prediction. The similitude and the differences between both of them are discussed. The Newton–Raphson numerical method respectively the minimisation of an error-function has been used for the numerical identification of the Yld96 and BBC2000 coefficients. The necking phenomenon was modelled by using the Marciniack–Kuczinsky (M–K) theory. In the present study an aluminium alloy sheet metal AA5XXX is considered. Yield surface shapes, yield stress and r -value directionalities of Yld96 and BBC2000 were investigated and compared with the experimental data. A successful correlation is observed between the experimental FLDs and the computed limit strains when, the shape of yield locus is described by Yld96 criterion respectively BBC2000 criterion and the hardening law represented by Voce equation.

Anisotropy of Sheet Metal

Due to their crystallographic structure and the characteristics of the rolling process, sheet metals generally exhibit a significant anisotropy of mechanical properties. The variation of their plastic behavior with direction is assessed by a quantity called Lankford parameter or anisotropy coefficient [4.1]. This coefficient is determined by uniaxial tensile tests on sheet specimens in the form of a strip. The anisotropy coefficient (r) is defined by

Anisotropy and Formability

r = \frac{{{\varepsilon _2}}}{{{\varepsilon _3}}}

Limit strains in the sheet metals by using the new Hill's yield criterion (1993)

(4.1) where e 2; e 3 are the strains in the width and thickness directions, respectively. Eq. 4.1 can be written in the form